[U-Boot] [PATCH 1/5] nand: Use common read function instead of verify_buf()

[Scott Wood]

On Thu, 2015-01-29 at 17:37 -0600, Peter Tyser wrote:
> On Thu, 2015-01-29 at 17:02 -0600, Scott Wood wrote:
> > On Tue, 2015-01-27 at 17:47 -0600, Peter Tyser wrote:
> > > Hi Scott,
> > > 
> > > 
> > > > > I waffled about removing it, but leaned towards leaving it in because:
> > > > > - I didn't want to change the existing U-Boot behavior for other
> > > > > users.  A google of 'u-boot "nand write"' shows a lot of examples that
> > > > > don't include verification of writes, and they should if we remove
> > > > > auto-verification.
> > > > 
> > > > How many configs actually enable this option?  I don't see many beyond
> > > > the FSL PPC boards (which are so full of copy-and-paste that it probably
> > > > wasn't deliberate).
> > > 
> > > Yeah, the majority are FSL 83xx and 85xx, with 2 or so random ARM boards.
> > > 
> > > > > - The reason it was removed in Linux was "Both UBI and JFFS2 are able
> > > > > to read verify what they wrote already.  There are also MTD tests
> > > > > which do this verification."  I thought U-Boot was more likely than
> > > > > Linux to use raw NAND writes without a filesystem, so leaving it in U-
> > > > > Boot made sense since the UBI/JFFS2 logic didn't apply as much here.
> > > > 
> > > > Right, though raw writes ought to be limited to blocks that aren't
> > > > written often enough to fail.
> > > > 
> > > > > - I didn't think a lot of people would know they have to explicitly
> > > > > verify NAND contents after a write, since they'd assume it was like
> > > > > other memories that aren't as lossy.
> > > > > 
> > > > > - The penalty of slightly different code from Linux and a small
> > > > > performance hit was worth the gain of auto-verification to me.  I
> > > > > viewed consolidating it into one small chunk of code as a happy medium.
> > > > 
> > > > The davinci patches show that there can still be driver dependencies
> > > > depending on what the driver overrides.  I'm not hugely opposed, but it
> > > > seems like it would be better to do it at a higher level (e.g. in
> > > > nand_util.c with a flag to enable, and either make support mandatory, or
> > > > if you try to use that command variant without support it fails rather
> > > > than silently not verifying).
> > > 
> > > That seems like a good idea.  How about:
> > > - Remove all CONFIG_MTD_NAND_VERIFY_WRITE references
> > > 
> > > - Add a new flag WITH_WR_VERIFY and have nand_write_skip_bad() in
> > > nand_util.c verify writes only when it is set.
> > > 
> > > - Update the calls to nand_write_skip_bad() in cmd_nand.c to include
> > > the new WITH_WR_VERIFY flag.  I'd vote to enable it for all boards,
> > > but let me know if you disagree.
> > > 
> > > That would make all "nand write" commands verify writes, with the
> > > exception of "nand write.raw".  Any opinion on if this should also
> > > be verified?  I only use it for development/testing, so don't have
> > > a strong opinion.
> > 
> > "raw" refers to the absence of ECC, and I'd rather not overload it to
> > mean "don't verify".  Should it also be possible to request non-raw
> > non-verified accesses?  Or should we always verify and wait until
> > someone complains about performance?
> 
> OK, I'll add verification to the "nand write.raw" functionality too.
> I'd lean towards always verifying and waiting until/if someone
> complains about performance.  I doubt many (any?) people are doing
> timing critical writes in U-Boot.  I think the argument that writes
> should be verified carries some water, and it'd be nice to not
> make the command arguments more complicated than they already are.
> 
> > What about DFU and other non-cmd_nand NAND accesses?
> 
> Are there other non-cmd NAND accesses other than DFU?  None jumped
> out at me.

There's ubi/jffs2/yaffs2, which are responsible for their own
verification, and a read-only access in compulab board code, but
otherwise maybe not.  The MTD interface is harder to grep for, though.

>   I'm not too familiar with DFU, but in theory the DFU
> programming utilities could already be doing their own verification.
> I took a quick look at the dfu-util source, and it doesn't appear
> to be doing its own.  I'd vote to verify the DFU writes too, since
> even more than "nand write" its performance shouldn't be very
> critical.  I'll break DFU verification out into a separate patch,
> and you or others can ACK or reject it then.
> 
> Let me know if the above sounds good and I'll make the changes.

Fine with me.

-Scott